I think validation is a good litmus test of whether you've done things properly, so if there are only a few minor problems, why not fix them and ensure your site will at least be understood correctly by browsers in the future (even if they do render things differently for other reasons)?
OTOH, for most projects, validation seems like a huge headache and if you can get things working across browsers, it's not worth spending an extra day/week+ on just validation.
